In certain surgical procedures various articles are implanted in the human body for a limited period of time. For example, when malignant growths are surgically removed from the body, it is sometimes desirable to implant small vials containing radioactive substances in the region where the malignancy was removed. This provides a localized low level of radiation which can be effective to destroy any remaining traces of malignancy in tissue which was not removed. After the radioactive implant has been in the patient for an effective period of time, the implant is surgically removed.
One complication that has arisen in the use of these radioactive implants is that, after the original incision has healed and possibly a small migration of the implant has occurred, the surgeon may find it difficult to locate the precise position of the implant without probing and causing unnecessary distress to the patient. To overcome this problem, it has been proposed to include a small object that will produce an image on an X ray screen or photographic film that will accurately locate the implant prior to surgical removal. The selection and design of such a device is subject to several complicating factors.
First, any substance that is implanted in the human body must be nontoxic and, for this reason, some of the more common materials that are known for their ability to absorb X rays, such as lead, cannot be considered. Second, the device should be made as small as possible to avoid unnecessary discomfort to the patient and this dictates that the locator device be made from materials that will be effective in only small amounts to leave an image on an X ray screen.
Accordingly, it is an object of this invention to provide an article for implantation in the human body which is nontoxic and which will leave a distinct image on an X ray screen or photographic film.
These and other objects of this invention are achieved by incorporating finely divided barium titanate in a film forming polymeric matrix. It has been found that small chips or wafers of such a material when implanted in an animal body, are effective to scatter X rays and leave an image on an X ray screen or film. Suitable polymers for use as a matrix material that are nontoxic and have good chemical resistance to body fluids include, for example, polyamides, polyolefins and fluorocarbons such as polytetrafluoroethylene.
To obtain good scattering of X rays in thin sections, e.g. from about 2 to 100 mils, it is important to work as much barium titanate into the polymeric matrix as it will accept as is consistent with maintaining good enough physical properties to enable handling and shape formation. This requires obtaining a thorough dispersing of barium titanate in the polymer. For example, with proper care, upwards of 50%, and as much as 60% by weight barium titanate can be worked into a nylon matrix. A thorough dispersion of the barium titanate throughout the nylon is also important in order to obtain reproducable and uniform X ray scattering.
To achieve the desired degree of dispersion, it is convenient to melt-mix the polymer and the barium titanate under conditions of intensive mixing such as may be generated in a twin screw compounding extruder. The melt-mixed material may then be extruded directly into a strip or sheet from which a convenient shape may be stamped. Alternatively, small diameter rods may be extruded, chopped and pelletized for subsequent extrusion into a desired shape at a more convenient time.
As the high filler loading of barium titanate is abrasive and dulls cutting tools, it has been found most convenient to extrude sheets of material in the desired thickness, to slit the sheets into strips, and stamp small diameter buttons from the strips. Buttons about 1/2 inch in diameter and about 1/16 of an inch thick have proven to be particularly useful. They are not so large as will cause distress to the patient, but are large enough to leave a clearly defined image on an X ray screen or film when implanted in animal tissue.